Dates are notorious for having many possible representations varying by locale and personal preference.
Provide a visual representation of dates in the familiar calendar format to facilitate people entering dates in a consistent format.
Date pickers provide a familiar calendar metaphor that resonates with most people and it helps them to think about dates in terms of where they are in the month and week. They help structure the date input in a way that your system can interpret reliably, thereby reducing validation errors caused by unrecognized date formats, and they are easier to use than multiple drop down choosers with, e.g., month, day, year.